The field of this invention relates generally to radio frequency (RF) modules for wireless communication units capable of reconfigurable phase shift combining and more particularly to RF transceiver modules comprising a reconfigurable duplexer.
Wireless communication units, e.g. portable radios, telephones, etc., are conventionally supported operation in a single radio frequency (RF) band, i.e. the operational band employed by the communication system. However, due to the rapid growth of mobile communications, there has been a comparable increase in the amount of spectrum that is required to support the various radio frequency standards, systems and services that are now available to mobile users. Furthermore, as radio frequency (RF) communication systems have evolved, there has been a recent trend for mobile communication devices to support communications in a plurality of RF bands, for example to support communications across a plurality of communication standards or geographical regions. Currently, a typical transceiver adapted to support a range of communications within, for example, a 3rd generation partnership project (3GPP™), may be required to support more than ten distinct frequency bands.
One approach to supporting a wide frequency range that covers many frequency bands or communication standards in a wireless communication unit is that many parallel transceiver circuits are required to be physically located within the wireless communication unit and/or one or more of the parallel transceiver circuits needs to be tunable to operate across multiple frequency ranges. A number of adverse effects follow from either of these solutions within a wireless communication unit, including requiring higher quality radio frequency (RF) components that are less prone to interference, providing higher performance, consuming minimal amounts of silicon area or module board area, etc. Furthermore, these solutions also increase design complexity in order to meet performance requirements simultaneously over all the supported frequency bands.
A typical mobile (wireless) communication unit (sometimes referred to as a mobile subscriber unit (MS) in the context of cellular communications or a user equipment (UE) in terms of a 3rd generation partnership project (3GPP™) communication system) contains an antenna often coupled to a duplex filter or antenna switch that provides some isolation between receive and transmit chains within the wireless communication unit. Of course these components in and around the antenna need to be able to operate across wide frequency ranges or be able to work in very close proximity with comparable/parallel RF circuits.
Furthermore, in such communications standards, the radio access network often employs technologies to facilitate more and easier communications, such as Frequency Division Duplexing (FDD) whereby a different set or band of receive frequencies and transmit frequencies are used for communications. Additionally, Time Division Duplexing (TDD) may be utilised, resulting in separate outward and return signals in the time domain. Furthermore, the access network may utilise different waveforms, signal modulation and coding schemes to differentiate between the different communications units.
A critical part of any FDD radio system is the front-end duplexer component, whose main function is to protect the receiver (Rx) from desensitization due to transmitter (Tx) noise emissions and transmitter power leakage impairments into the communication unit's receive frequency band. Such impairments may be due to one or more of: intermodulation, cross-modulation with out-of-band blocker, and receiver local oscillator phase noise reciprocal mixing.
In full-duplex radio systems, the receive frequency (frx), and transmit frequency (ftx), are typically different and they are separated by a specific ‘duplex frequency’ (fdpx)=frx−ftx.
One example of a known front end architecture 100, for example employing SAW based duplexers 150 is illustrated in FIG. 1. In SAW based duplexers, the concept of phase shifting is used to transform a low-Z notch at a stop band frequency in one duplexer branch into high-Z level at the antenna input in order not to minimize insertion loss at the passband frequency in the other duplexer branch. This provides isolation between both paths whilst connecting them at antenna side. In FIG. 1, therefore, an antenna 102 is operably coupled to an upper radio frequency path 104, say of a receiver, and a lower radio frequency path 106, say of a transmitter. A high-pass (HP) type phase shift network 108 is introduced into the Rx path, and is arranged to transform a low-Z notch at the transmit centre frequency into a high-Z level at the antenna input 102. Similarly, a low-pass (LP) type phase shift network 110 is introduced in the Tx path, and is arranged to transform the low-Z notch at the receive centre frequency into a high-Z level at antenna input. A high pass (HP) phase shift network 108 or low pass (LP) phase shift network 110 is used specifically in a duplexer branch depending on which side the desired passband frequency in that branch is relative to the stopband frequency. Frequency selectivity in the phase shifting networks also provides further attenuation in far-out stopband frequencies. The respective filter responses are performed by surface acoustic wave (SAW) filters 112, 114, which have ports 116, 118 that are matched to the respective receiver and transmitter chains.
Typically, in all second generation (2G) and third generation (3G) cellular FDD bands, frx>ftx and fdpx=frx−ftx is small percentage (typically <10%) of frx and ftx. However, in fourth generation (4G) communication frequency bands, there are some additional frequency bands that have been provided in which ftx>frx, which is denoted as a reverse duplex case 150 (and where fdpx<0).
In 3G/4G cellular systems, the number of FDD frequency bands that are supported increases depending on the number of in-band and outbound roaming bands that are required. In addition, the need to support multiple-input, multiple-output (MIMO) communications can add more stress on the RF front end complexity. Each of these performance requirements and associated technical complexities and corresponding circuits/components effectively limits the number of duplexers that can be supported in a single radio platform. Compacting the number of these duplexers without impacting performance is highly desirable because of the direct impact on radio frequency (RF) front-end solution size and cost reduction
A duplexer for wireless communication units uses two resonant structures that exhibit a bandpass frequency response centered at both the receive frequency band and transmit frequency band center frequencies. The center frequencies are typically located very close to each other and are connected at an antenna port occasionally through a phase combining network. The duplexer's receive frequency bandpass response helps to reject strong out-of-band blockers and minimize the known coexistence problem with signals from nearby communication units. The duplexer transmit bandpass response helps to suppress transmit out-of-band noise emissions, transmitter harmonic emissions, and reverse intermodulation of the transmit signal with interfering signal reaching the transmitter via the antenna.
A phase matching network ensures that each filter's impedance at the antenna port is exactly matched at its own passband and with an almost ‘open’ (i.e. exhibiting a high impedance (Z)) at the other side's (converse Rx/Tx) in filter stopband frequency. If the phase matching network is not ‘open’, a phase shifter is generally used to rotate the impedance of the phase matching network to an equivalent ‘high Z’.
For 4G reverse duplexing cases (where ftx<frx), the order is reversed as far as the HP/LP phase shifting combiner topology. Therefore, a low-pass/high-pass (LP/HP) phase shifting combiner may be used instead, in order to connect to the respective Rx and Tx paths.
In a case of a tunable duplexer that requires the tuning over different cellular bands (e.g. covering at least LTE cellular bands in the 700-1000 MHz range) as well as being able to handle both normal and reverse duplex cases, it is not sufficient to tune the elements of a phase shifting network, such as phase shift networks 108, 110 of FIG. 1.
Thus, a need exists for an improved radio frequency front-end architecture that supports, for example, both normal and reverse duplexing in a cost-effective and manageable complexity manner.